How to properly expand multiple variables in ansible when: statement?

I have the following play working correctly as expected:

- name: Identify next available hostname    
  ansible.builtin.set_fact:      
    NEXT_HOSTNAME: "{{ HOSTNAME_CONVENTION }}{{ '{:02d}'.format(item) }}"    
  loop: "{{ range(1, 99) | reverse | list }}"
  when: "'host-name{:02d}'.format(item) not in EXISTING_HOSTNAMES

Where EXISTING_HOSTNAMES is a list of non-sequential hostnames already assigned and HOSTNAME_CONVENTION is set to 'host-name'.

My goal is to substitute out the "host-name" text in the when statement with the HOSTNAME_CONVENTION variable similar in manner to how the "NEXT_HOSTNAME" variable value is being built.

Substituting out the "when" line to

when: "'HOSTNAME_CONVENTION{:02d}'.format(item) not in EXISTING_HOSTNAMES

allows the play to run, but it does not correctly evaluate, as I expect it to skip host-name01, host-name02, and host-name04 since they are already assigned, whereas with the static "host-name" text on the when line, it does correctly skip host-name01, host-name02, and host-name04.

Substituting out the "when" line to

when: "HOSTNAME_CONVENTION'{:02d}'.format(item) not in EXISTING_HOSTNAMES

fails completely with "template error while templating string: expected token 'end of statement block', got 'string'"

Solution

Declare the host convention, start, stop, format, and index

  hc: host-name
  start: 1
  stop: '20'
  frmt: "%0{{ stop|length }}d"
  index: |
    {% for i in range(start, stop|int) %}
    {{ frmt|format(i) }} {% endfor %}

give

  frmt: '%02d'
  index: |-
    01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19

Create all hostnames

  hn: "{{ [hc] | product(index|split) | map('join') }}"

gives

  hn:
  - host-name01
  - host-name02
  - host-name03
  - host-name04
  - host-name05
  - host-name06
  - host-name07
  - host-name08
  - host-name09
  - host-name10
  - host-name11
  - host-name12
  - host-name13
  - host-name14
  - host-name15
  - host-name16
  - host-name17
  - host-name18
  - host-name19

Given the list of existing hostnames. For example,

  existing_hostnames: [host-name01, host-name02, host-name04]

remove them from the list of all hostnames

  result: "{{ hn | difference(existing_hostnames) | sort }}"

gives what you probably want

  result:
  - host-name03
  - host-name05
  - host-name06
  - host-name07
  - host-name08
  - host-name09
  - host-name10
  - host-name11
  - host-name12
  - host-name13
  - host-name14
  - host-name15
  - host-name16
  - host-name17
  - host-name18
  - host-name19

^{Example of a complete playbook for testing
- hosts: all

vars:

hc: host-name
start: 1
stop: '20'
frmt: "%0{{ stop|length }}d"
index: |
{% for i in range(start, stop|int) %}
{{ frmt|format(i) }} {% endfor %}
hn: "{{ [hc] | product(index|split) | map('join') }}"
existing_hostnames: [host-name01, host-name02, host-name04]
result: "{{ hn | difference(existing_hostnames) | sort }}"

tasks:

- debug:
var: hc
- debug:
var: frmt
- debug:
var: index
- debug:
var: hn
- debug:
var: result

Q: Addition of the first filter in the result variable.
A: It's simpler to concatenate the result if you want the first item only
existing_indexes: "{{ existing_hostnames | map('replace', hc, '') }}"
result: "{{ hc }}{{ index|split|difference(existing_indexes)|first }}"

give
existing_indexes:
- '01'
- '02'
- '04'

result: host-name03

Example of a complete playbook for testing
- hosts: all

vars:

hc: host-name
start: 1
stop: '20'
frmt: "%0{{ stop|length }}d"
index: |
{% for i in range(start, stop|int) %}
{{ frmt|format(i) }} {% endfor %}

existing_hostnames: [host-name01, host-name02, host-name04]
existing_indexes: "{{ existing_hostnames | map('replace', hc, '') }}"
result: "{{ hc }}{{ index|split|difference(existing_indexes)|first }}"

tasks:

- debug:
var: existing_indexes
- debug:
var: result}